Visual simplification of user interfaces

There was a time, starting somewhere in the late 90's, and continuing for over a decade, of operating systems and programs using fancier and fancier looking graphics for GUI elements. This was true in all three major operating systems, and the majority of applications followed suit.

However, in later years, for some reason, there has been a trend into the opposite direction. Sometimes to the ridiculous extreme.

Consider, for example, the window decorations in Windows 7 vs. those of Windows 10:

The change into the opposite direction is just outright ridiculous. It goes so far as to be actually detrimental to usability. Of course every single fancy graphical effect is gone, and symbols have become nothing but one-pixel-wide straight lines, but that's not all.

In Windows 10 there is no difference in coloration between the title bar of the active window vs. an inactive window; it's always just pure white. (Applications are supposed to define their own colorations, which no application currently does, of course. Even then, it's just incomprehensible why sensible defaults can't be used.) Also, there is no border for the buttons (a trend that's absolutely detrimental, although it started well before Windows 10. However, now it has crept itself even into title bar buttons.)

Also notice that there is zero visual difference between menu titles (which are interactive elements) and the title of the window. They are all the same. You simply have to know that those are interactive menu buttons, as there is no actual visual distinction otherwise. (Also, in Windows 10 the "click&drag" areas around the borders, which you can use to resize the window, are invisible, outside the visible border of the window. Again, there is no visual indication of where the window could be resized with the mouse.)

The fact that window decorations are so utterly simplistic actually makes it sometimes very hard to use. When you have several windows open, one on top of another, all of them consisting of plain white background and one-pixel-wide borders, it becomes visually hard to distinguish between the windows. Sometimes it's even hard to see where the title bar of one window is. Needless to say, this wasn't a problem in Windows7.

Windows is not the only operating system embracing this trend. In the late 90's and the first decade of the 2000's, Mac OS X went into the direction of making fancier and fancier GUI designs, with all kinds of shiny graphical effects. Now they, too, have been going into the other direction. For example, just compare the upper left corner of Finder in older versions of Mac OS X vs. the current version:

Operating systems are not the only ones doing this. As a curious example, compare how the Google logo has changed over time. There has been a continuous trend towards simplicity there as well. Perhaps the most iconic version was the one used for longest, ie. the one used between 1999 and 2010. Then they simplified it, and simplified it, and simplified it... until the current version (from September 1, 2015) is just ugly. Not only are the shadows and lighting effects gone, but now even the font is a really ugly simplistic sans-serif that removes anything fancy even in the outline of the letters.

I really don't understand where this trend is coming from, and why everybody is embracing it. Sometimes you even see it in casual mobile games, with some games using amazingly simplistic graphics that look like they have been done in MS paint. (And, quite incredibly, a few of these games are some of the best-selling ones... which of course means thousands of copy-cats, who think that it's the simplistic graphics that sell the game.)